1. Field of the Invention
The present invention relates to a content processing apparatus and encryption processing method which process contents.
2. Description of the Related Art
There are a various kinds of contents. They include contents which do not require protection and those which require it. The latter contents are protected by several forms. One of the forms is “copy is inhibited, but move is permitted”.
Examine a case in which a content whose “copy is inhibited but move is permitted” can be moved from a first recording medium to a second recording medium.
For example, assume a move procedure of copying the content from the first recording medium to the second recording medium in the first step and then deleting the content from the first recording medium in the second step. If this move procedure correctly finishes, no problem rises. However, if the procedure is interrupted between the first step and the second step, a “state in which the content is copied from the first recording medium to the second recording medium” occurs.
Occurrence of this state poses a problem when copy is inhibited. Hence, a means for preventing occurrence of this state is necessary.
There is a method of encrypting a content and recording, in a recording medium, a pair of the encrypted content and a content key necessary for decryption.
In this case, even when the content whose “copy is inhibited but move is permitted” is copied from the first recording medium to the second recording medium, copy protection of the content can actually be achieved by moving the content key from the first recording medium to the second recording medium without copy.
For example, assume a move procedure of copying the encrypted content from the first recording medium to the second recording medium in the first step and then copying the content key from the first recording medium to the second recording medium and deleting the content key from the first recording medium in the second step. If this move procedure correctly finishes, no problem rises. Even if the encrypted content is not correctly transmitted due to a poor transmission path quality, or a recording error occurs in the second recording medium, the encrypted content can be resent any number of times. It is also possible to interrupt the move processing and restore the initial state of the move processing. However, if it is possible to restore the initial state in the second step, a “state in which the content key is copied from the first recording medium to the second recording medium” can be caused by intentionally disconnecting a response or altering a response. To prevent this, it is necessary to delete the content key from the first recording medium early. However, if the content key is not recorded in the second recording medium in the second step, a “state in which the content key is stored in neither the first recording medium nor second recording medium” occurs because resend of the content key is possible.
If this state occurs, the encrypted content cannot be decrypted by using either of the first recording medium and second recording medium. Hence, occurrence of this state also poses a problem.
An attack called “save/restore attack” is known. A technical specification under the technical license of 4C Entity LLC (“Content Protection for Recordable Media Specification, Portable ATA Storage Book” (http://www.4Centity.com/)), and the like have disclosed a technique of preventing the save/restore attack by using dynamic information for encryption processing.
However, the technique disclosed in this technical specification cannot avoid the problem.
As described above, conventionally, it is impossible to safely and reliably move, between recording media, a content key necessary for decryption of a copy-protected content.